A high electron mobility transistor (HEMT) is a field effect transistor incorporating a junction between two materials with different band gaps (i.e., a heterojunction). Gallium nitride (GaN) HEMTs have attracted attention due to their high-power performance. In type III-nitride HEMT devices used in power applications there is a conflicting design trade-off between the on-state resistance and breakdown voltage (BV). Since the relation between the BV and on resistance is at least quadratic, improvement in the BV for a given drift region length results in a significant improvement in the FOM of the device, defined as BV2/Ron.
In the prior art type III-nitride HEMT devices have a uniform 2DEG density which results in a peak electric field under or near the gate region. The electric field distribution tends to be closer to a triangular shape than to the desired trapezoidal shape which reduces the breakdown voltage per unit drift region length of the device. The use of field plate and multistep field plates are some of the techniques that are used to improve the electric field distribution. However, field plates typically result in multiple peaks and suffer from less than an ideal flat field distribution, and may exhibit a saw tooth profile. Field plates also add to the gate to drain capacitance. In addition, process complexity and cost typically increase with the number of field plate steps.
U.S. Pat. No. 7,038,253 to Furukawa describes a GaN based device on silicon (Si) technology which uses a uniform 2DEG profile in the drift region. Because of the absence of any field shaping technique in the Furukawa device, the breakdown voltage and dynamic on resistance from drain to source is limited by a localized increase in the electric field under the gate region thus requiring over design of the device which degrades the figure of merit (FOM) that such a device can achieve.
In “High Breakdown Voltage AlGaN/GaN HEMTs Achieved by Multiple Field Plates” by H. Xing et. Al, a field shaping technique that uses multiple field plates is described to improve the electric field distribution. However, multiple field plates do not achieve a uniform electric field, may have a saw tooth type distribution, and introduce gate to drain capacitance. Implementing such a device structure also increases device complexity and cost.
What is needed is a significant improvement in the FOM in HEMT devices, and in particular an improvement in the breakdown voltage for a given drift region length, so that the FOM of the device, defined as BV2/Ron, improves. The embodiments of the present disclosure answer these and other needs.